The fluorescent Mg^<2+> indicator, furaptra, was injected into single frog skeletal muscle fibers, and the indicator's fluorescence signals were measured at rest and analyzed in terms of the intracellular free Mg^<2+> concentration ([Mg^<2+>]_i)(17ﾟC). Based on the fluorescence excitation spectrum of furaptra, the calibrated [Mg^<2+>]_i level averaged 0.54 mM, if the value of dissociation constant (K_D) obtained in vitro (5.5 mM) was used. However, if the indicator reacts with Mg^<2+> with a two-fold larger K_D in myoplasm, as previously suggested for the furaptra-Ca^<2+> reaction (Konishi et al, 1991, J. Gen. Physiol. 97 : 271-301), the calculated [Mg^<2+>]_i would average 1.1 mM. Thus, the value 1.1 mM Probably represents the best estimate from furaptra of [Mg^<2+>]_i in resting muscle fibers. Extracellular perfusion of muscle fibers with high Mg^<2+> concentration solution or low Na^+ concentration did not cause any detectable changes in [Mg^<2+>]_i within 4 min.Ventricular myocytes enzymatically isolated from rat heart were loaded with furaptra. The fluorescence signals were measured from single myocytes (32ﾟC), and analyzed as in the skeletal muscle fibers The estimated [Mg^<2+>]_i for the resting single myocytes was between 0.8-1.3 mM. Superfusion with a high extracellular Mg^<2+> concentration caused a very slow and small elevation in [Mg^<2+>]_i over a period of hours. Other experimental interventions, including a low extracellular Na^+ concentration, isoproterenol and CO_2 acidosis, did not cause detectable change in [Mg^<2+>]_i, whereas the application of a mitochondrial uncoupler caused a rapid and large increase in [Mg^<2+>]_i.The results suggest that the [Mg^<2+>]_i level of around 1 mM is similar in different muscle types (skeletal and cardiac muscles). It is also suggested that [Mg^<2+>]_i is highly maintained at around 1 mM in both skeletal and cardiac muscles, unless intracellular ATP is depleted.